1. Field of the Invention
The present invention relates to a color processing apparatus and a color processing method and, more particularly, to a color processing apparatus and a color processing method for correcting a profile.
2. Description of the Related Art
To match colors between various image input and output apparatuses, a profile such as an International Color Consortium (ICC) profile is used. The profile includes an A2B tag with a lookup table (LUT) to convert a device-dependent signal value into a device-independent signal value, and a B2A tag with a LUT for converting a device-independent signal value into a device-dependent signal value. Note that examples of the device-dependent signal value are an RGB signal value and CMYK signal value. The device-independent signal value is a signal value (PCS value) in a profile connection space (PCS), for example, a CIELAB value.
To create the LUT of the A2B tag, a color chart including a plurality of patches for which signal values are known is displayed on a monitor, or printed by a printer, and the color value (for example, the CIELAB value) of each displayed or printed patch is measured, thereby acquiring a correspondence between the signal value and the color value (PCS value). With interpolation processing based on the acquired correspondence, the LUT of the A2B tag for converting a signal value into a PCS value is created for the whole range of the signal value. On the other hand, to create the LUT of the B2A tag, grid points are defined in the PCS, a signal value corresponding to the PCS value of each grid point is obtained by executing an interpolation operation.
Color conversion using the thus created profile enables to match colors between image input and output apparatuses. The color gamut of the image input apparatus is different from the range (to be referred to as a “color reproducible range” hereinafter) of color which can be reproduced by the image output apparatus, and the color reproducible range of the image output apparatus is usually smaller than the color gamut of the image input apparatus. Consequently, the image output apparatus cannot reproduce a color in a color gamut outside the color reproducible range. It is, therefore, necessary to map the color gamut of the image input apparatus into the color reproducible range of the image output apparatus. The color reproducible ranges of a monitor and printer both of which are image output apparatuses are different from each other, as a matter of course. Therefore, the color impression of an image displayed on the monitor is different from that of an image output from the printer, and the tonality of the printed image is lower than that of the displayed image.
To deal with these problems, there is proposed a method of improving the color reproduction accuracy of a profile. For example, the profile is used to output a color chart, and the amount of correction is determined based on the colorimetric value of each patch of the output color chart. Alternatively, the user visually evaluates the color chart to determine the amount of correction. The profile is then corrected based on the amount of correction.
The above method requires heavy labors for printing and measurement of the color chart, and thus it takes time to correct the profile. Furthermore, although visual evaluation eliminates the need for measurement, another problem arises. That is, the profile is corrected in the same color space as that in which the profile has been designed, for example, a standard color space such as a CIELAB color space, but the standard color space is not a uniform color space for human appearance (to be referred to as a “non-uniform color appearance space”). Correction intended by the user, therefore, may not be reflected on a result, and thus an appropriate profile cannot be always obtained by one correction operation.
FIG. 1 is a graph obtained by plotting color discrimination thresholds (see literature 1) for 25 colors created by MacAdam in the CIELAB space. Note that FIG. 1 is a graph obtained by magnifying the color discrimination thresholds (to be referred to as “MacAdam ellipses” hereinafter) by 10 times, and plotting only chromaticity information on an a*b* plane for descriptive convenience.
Literature 1: D. L. MacAdam “Visual sensitivities to color differences in daylight” Journal of the Optical Society of America, Vol. 32, No. 5, pp. 247 to 274, in May, 1942
Each ellipse shown in FIG. 1 indicates a range where a human recognizes color as the same one. The area of an ellipse for color with low chroma is relatively small, and the area of an ellipse for color with high chroma, especially for blue or green, is very large. That is, a human can discriminate colors with low chroma even though a distance within the color space is short. For blue or green with high chroma, however, it is difficult to discriminate colors even though a distance within the color space is long.
FIG. 2 shows a case in which a profile has been designed so that a chroma value C* is plotted at a regular interval in a non-uniform color appearance space. In a low chroma range, there is a big difference between the chroma appearances of colors (patches), and it is possible to identify the different colors (patches). In a high chroma range, however, there is a small difference between the chroma appearances of colors (patches), and it is difficult to identify the different colors (patches). It is, therefore, very difficult to correct the profile in the non-uniform color appearance space.